It is well known in the art that electronic equipment such as semiconductor chips, portable digital assistants (PDAs), personal computers (PCs), etc., generate increasing amounts of heat as the density of components increases and/or the size of the device becomes smaller. The thermal management problem is becoming most urgent and is receiving increasing attention in the field.
All sorts of cooling implementations are being advanced to assist in the regulation of temperature for such devices particularly those subject to high heat flux. Current prior art efforts have reached a limiting point at which electronic devices can generate heat at a faster rate than can be transmitted through the enveloping surfaces. Current prior art devices include heat pipes, diamond substrates, circulating fluid heat sinks, and many others which may be subject to a heat flux as much as 1,000,000 W per m2. Unfortunately, no system has yet been offered which sufficiently regulates temperature of, for example, semiconductor integrated circuit chips. Instead, control circuits are often provided to sense temperature and to reduce operating speeds to maintain temperatures within acceptable limits. The design and specification of the size, operational level and speed for portable devices and other high flux generating devices is constrained by thermal effects. Accordingly, a significant problem faced by manufacturers of portable electronic equipment and/or densely packed equipment is temperature regulation and subsequent heat removal.
In accordance with the principles of this invention, thermal regulation and heat removal from, for example, an electronic heat source, is greatly enhanced by moving small amounts of fluid past the hot spots at a rate and at a pressure sufficient to insure that no change of phase occurs within the fluid. Regulation of temperature and heat transfer for systems generating a heat flux in excess of 1,000,000 W per m2 and up to 10,000,000 W. per m2 can be realized. One of the major aspects of the invention is based on the recognition that only a supracritical fluid, at a sufficiently high mass flux, would be effective in transfer and removal of heat from such an electronic source. That is because such a source produces such an extremely high flux condition that prior fluid heat removal systems experience some form of flow restriction akin to vapor lock and thus fail to consistently remove heat at a sufficient rate. The reason for this is that these prior art heat removal systems have working fluids which either wholly or partially vaporize and thereby may cause catastrophic failure of the system. Also, these prior art heat removal systems lack a device designed for a heat source which produces the kind of high heat flux condition currently being developed and thus fail to remove heat at a sufficient rate to ensure proper operating temperature regulation. Thus the invention comprises devices which allow supracritical state working fluids to efficiently and effectively regulate the transfer of heat to or from a high heat flux surface.
In a first embodiment, supracritical fluid is circulated through a device consisting of pathways for the working fluid which allow for the direct impingement of the working fluid upon the high heat flux surface. The device allows for sufficient mass flux, very brief residence time of the working fluid on the hot surface, and continuous, rapid fluid replacement for constant heat removal.
In a second embodiment, a porous heat sponge made of, for example, POCOFOAM(trademark) or sintered graphite is included against the heat generating surface. This device circulates a supracritical fluid, for example CO2 above its critical pressure of 1071 psia, and in such a manner that the mass flux through the porous sponge is sufficient to efficiently and effectively assist in the transfer of heat to a relatively low heat flux area from a high heat flux-surface. Research literature indicates that a mass flux greater than 500 kg per sec-m2 is needed.
A third embodiment includes such a device as in embodiment one along with the structural sponge envisioned for embodiment two. These items, connected either in series or in parallel, constitute a system wherein pure supracritical working fluid circulates in contact with surfaces of high to very high heat flux (1,000,000 W per m2 to 10,000,000 W per m2 or a combination of supracritical and subcritical working fluid. This embodiment has the ability to take advantage of any cooling arising from the evolution of a gas phase below the critical pressure in a selected parts of the system.
In all embodiments, the constraints imposed by high flux conditions are overcome and heat removal proceeds at rates unrealized by prior art heat removal and thermal regulation systems. Also the flow of working fluid through the devices envisioned in embodiments 1 through 3, is actively controlled in such a manner so as to maintain required levels of mass flux, temperature and pressure anywhere within the system.
Supracritical working fluid at a sufficient mass flux is required for proper functioning of the devices envisioned in all embodiments herein when heat is to be removed from a surface that generates a high heat flux (1,000,000 W per m2 to 10,000,000 W per m2). When the working fluid is in a supracritical state (herein meaning above the critical pressure and/or at the critical pressure of the working fluid). there is no opportunity for the fluid to form a vapor/liquid interface (vapor lock) which would greatly lower the ability of the high heat flux surface to transfer heat to the adjacent flowing work fluid. It is the formation of vapor lock at the vapor/liquid interface on or near the surfaces generating or receiving high heat flux, that can cause catastrophic failure of the system. More specifically, when the working fluid is in a supracritical state (herein meaning at/above/or below the critical temperature and above the critical pressure and/or at the critical pressure of the working fluid) there is no opportunity for the fluid to form a vapor-liquid interface (vapor lock) which could block further fluid flow or heat transfer.
An additional benefit of having supracritical working fluid occurs if the system is at or very near the critical temperature. At this condition, the enthalpy of the working fluid is enhanced due to the physical properties of the supracritical working fluid. Enthalpy as a function of temperature with the pressure constant shows a distinct jump from its normal monotonic rise. This notch arises from the natural evolution of the supracritical working fluid""s density from a dense to a highly less dense state that occurs as the temperature rises within several degrees of the critical temperature. For example, CO2 has a critical pressure of 7.39 MPa and a critical temperature of 31.1 C. At a pressure of 7.5 Mpa and between 25 C. and 40 C., CO2 there is a enthalpy difference of 154 kilojoules per kilogram CO2. Whereas between xe2x88x9225 C. and 25 C. the difference in enthalpy is 121 kilojoules per kilogram CO2 and between 40 C. and 90 C. the difference in enthalpy is 91 kilojoules per kilogram CO2. Thus within a small temperature range from 25 C. to 40 C. the ability of the working fluid to hold and transmit heat is greatly enhanced.
In still further embodiments, any one of the above embodiments is coupled with a cooling system comprising a novel arrangement of concentric cylindrical, (thermoelectric) coolers with the (interstitial) spacings between the coolers filled with porous heat sponge material through which supracritical fluid can be circulated.
The various embodiments herein are effective to maintain electronic devices at a preset temperature and avoid the dangers of destructive thermal runaway without the necessity of reducing operating speed and/or power input or output.